Methodical study of the chemical activation of Kraft lignin with KOH and NaOH

Abstract

A commercially available Kraft lignin was chemically activated with two alkaline hydroxides, viz NaOH and KOH, using different preparation conditions. The activation was made at various temperatures, hydroxide to lignin mass ratios, activation times, flow rates of inert gas, and heating rates. The resultant activated carbons were characterised in terms of BET surface area, total, micro and meso-pore volumes, average pore width, carbon yield and packing density. The influence of each parameter of the synthesis on the properties of the activated carbons is discussed, and the action of each hydroxide is methodically compared. It is the first time that so many preparation parameters and so many pore texture characteristics are simultaneously considered for two closely related activating agents of the same lignin precursor. Whatever the preparation conditions, it is shown that KOH leads to the most microporous materials, having surface areas and micropore volumes typically 1.5 and 1.2 times higher than those obtained with NaOH, which is in agreement with some early works. However, the surface areas and the micropore volumes obtained in the present study are much higher than in previous studies, up to 3100 m²/g and 1.5 cm³/g, respectively, using KOH. The thorough study of the way each preparation parameter influences the properties of the final materials brings insight into the activation mechanisms. Each time it was possible, the results of lignin chemically activated with hydroxides were compared with those obtained with anthracites: explanations of similarities and differences were systematically looked for.

Introduction

Lignin is a mass-produced waste of the paper industry and, as such, is generally used for its fuel value. However, an ever-growing number of research works are carried out in order to bring added value to this material. Lignin is becoming frequently accepted as a suitable chemical reagent for formulating new adhesives [1], [2], [3], [4], [5], as a filler in polymer mixtures [6], and as a promising precursor of carbonaceous materials [7], [8], [9], [10], [11], [12], [13], [14]. Other possible applications are reviewed in [15]. Concerning the preparation of carbons, lignin is particularly advantageous because of its high phenolic content, leading to higher carbon yields than those obtained from the two other main macromolecular compounds of biomass: cellulose and hemicellulose [16], [17]. Hence, getting almost pure lignin is really interesting for preparing carbons, and especially activated carbons.

In the present work, the action of two alkaline hydroxides classically used for chemical activation of various precursors was investigated thoroughly with one kind of commercially available Kraft lignin, once the latter was demineralised. All the parameters of the active carbon synthesis which could be varied were investigated, namely activation temperature, mass ratio hydroxide/lignin, flow rate of inert gas, activation time and heating rate. The influence of each of them was systematically correlated to the pore structure of the corresponding activated carbon. From this work, the suitable preparation conditions of the material having given desired porous characteristics could be identified, and insight into the activation mechanisms was obtained.

Section 2 of the present paper deals with the intrinsic features of the lignin; the experimental conditions by which it was demineralised and activated are described, and the way the resultant active carbons were characterised is also detailed. The pore volumes and the related properties: mean pore size, surface area, and also the carbon yield and the packing density obtained with the 2 activating agents are systematically compared and discussed in Section 3.